Silicon carbide is well known for its high strength, hardness and abrasion resistance. Consequently, it is employed in many applications requiring these properties. Some of the applications in which hardness and abrasion resistance are critical include cyclone separators for mineral processing, burner liners for particulate coal-fired electric power plants, etc. Although these products and processes could probably utilize substantially pure silicon carbide, such products are not available in all the shapes and sizes of interest and are difficult and expensive to produce. Consequently, for many applications, reaction-bonded silicon carbide is employed.
Reaction-bonded silicon carbide comprises a discontinuous silicon carbide grain phase held together within a continuous bond phase matrix produced in situ from the reactants. Bond phases typically found in reaction-bonded silicon carbide include, e.g., silicon nitride, silicon oxynitride and SiAlON.
Silicon carbide bonded with silicon nitride, Si.sub.3 N.sub.4, yields refractory articles with good abrasion-resistance, and such products have been commercially available for a number of years. For example, U.S. Pat. No. 2,752,258 discloses the use of silicon nitride as the bond phase for silicon carbide. In this disclosure, the silicon carbide grain is held together by intimately mixing it with silicon metal powder and water to produce a mixture moldable into a green body, and then firing the shaped green body in a non-oxidizing, nitrogenous atmosphere at the temperature and for the period of time necessary to convert substantially all the silicon metal to silicon nitride.
U.S. Pat. Nos. 2,618,565, 2,636,828 and 3,206,318 disclose the use of a fluoride, iron powder, vanadium metal or compounds containing vanadium, respectively, as a catalyst for the conversion of silicon metal to silicon nitride in the nitridation reaction. U.S. Pat. No. 4,990,469 describes the production of a silicon nitride-bonded silicon carbide by nitriding a slip castable mixture of silicon carbide, silicon, alumina and iron oxide.
The nitridation reaction between silicon metal, which is a solid at the usual firing temperature, and gaseous nitrogen is heterogeneous, in that the reactants are in separate phases, and the rate of reaction can be determined by the rate of nitrogen diffusion or transport into the solid. Consequently, the composition and physical properties of the reaction-bonded product may be expected to depend to some extent upon variables such as the particle size of the silicon carbide and the porosity of the green body.
The heterogeneous nitridation reaction can also be employed to yield bond phases other than silicon nitride. For example, reaction-bonded silicon carbide in which the major component of the bond phase is silicon oxynitride, Si.sub.2 ON.sub.2 is produced by nitriding a mixture including particulate silicon carbide, silicon metal powder, and an oxygen source. The resultant refractory articles have very good abrasion resistance. These products are available from The Carborundum Company, Niagara Falls, N.Y., as CAST REFRAX.RTM. refractories.
SiAlON is yet another bond phase which is useful for making reaction-bonded silicon carbide with good abrasion resistance. "SiAlON" is an acronym coined to represent the stable solid solutions which result from the replacement of silicon and nitrogen atoms in compounds such as silicon nitride and silicon oxynitride with aluminum and oxygen atoms, respectively. Since some, but not all the silicon and nitrogen atoms are replaced, SiAlON represents, not a single substance, but a range of compositions representing different degrees of replacement. .beta.'-SiAlON is obtained from .beta.-silicon nitride, O'-SiAlon from silicon oxynitride.
SiAlON-bonded silicon carbide can be produced by nitriding a mixture of silicon carbide grain, silicon, an aluminum source, and an oxygen source. For example, U.S. Pat. Nos. 4,243,621; 4,578,363 and 5,302,329 disclose the production of .beta.'-SiAlon-bonded silicon carbide. U.S. Pat. No. 4,506,021 discloses O'-SiAlon ceramic products.
It is known that reaction-bonded silicon carbide articles, including those commercially available, have properties which can depend upon, not only the chemical composition, but also upon the method of fabrication, the particle size distribution in the raw batch, and the porosity of the green body, unless the green body is quite thin. The properties of the ceramic body are believed to be primarily the result of the fact the rate-determining step in the nitridation reaction is the rate of nitrogen gas diffusion into the green body, as pointed out above. Chemical kinetics, rather than thermodynamics, is known to control the outcome of many heterogeneous chemical reactions. On this basis, the following is offered as a nonbinding explanation of how this affects the products obtained from heterogeneous nitridation processes.
The nitriding reaction proceeds from the surface to the interior core of a green body as firing is initiated and continued. This progression is thought to require diffusion of nitrogen gas through voids or pores in the green body, i.e., diffusion and reaction rate depend upon the porosity of the green body. As the nitridation proceeds inward from the surface of the green body, some of the pores near the surface probably become nitrided but remain filled with nitrogen atoms whose progress further into the interior is then blocked by the nitridation products. Thus, the number of voids available for further nitrogen infiltration, diffusion and nitridation decreases.
As a result, the rate of reaction is reduced, and there is most likely unreacted silicon left proceeding from the surface further into the body. This is especially evident in cases in which the green body has a low amount of porosity to start with, particularly at the surface. In addition, the nitridation reaction is exothermic, which introduces additional complications affecting both the nitrogen diffusion rate and the inherent rate of the nitridation reaction.
The gradation in nitridation from the surface into the core of the article becomes even more significant if a plaster mold is used to produce the green body from a slip or other water-containing raw batch. Cast green bodies yield fired articles which are more dense, i.e., less porous, at the surface which contacts the plaster than in the core, because of the capillary action of the plaster at the surface. The plaster tends to draw the water out of the surface first, and transport of water from the interior to the surface to restore equilibrium is impeded in the solid green body. Evaporation of the residual water when the green body is dried and fired leads to additional pores. The lower porosity at the surface of the green body impedes diffusion of nitrogen gas into the body, retards the nitridation reaction, and causes a surface "skin" to be present on the fired reaction-bonded silicon carbide article.
As a result of these nitridation problems, most of the commercially available reaction-bonded silicon carbide products have properties which are not uniform throughout the article. They either have very good abrasion resistance until the "skin" wears through or have marginal abrasion resistance throughout the article. In addition, many of the reaction-bonded silicon carbide products offer relatively poor oxidation resistance in that the abrasion resistance of the product rapidly deteriorates upon exposure of the product to oxidizing conditions at elevated temperatures.
The ideal wear-resistant refractory article should first have superior abrasion resistance. The abrasion resistance should remain high even when the article is exposed for a prolonged period of time to oxidizing conditions at elevated temperatures. In addition, the abrasion resistance of the article should be high, not only within the surface skin, but throughout the material. Indeed, uniformity in both chemical composition and physical properties throughout the refractory article, regardless of its size or shape, is a long sought, but seldom attained goal.